In the oil and gas industry, seismic prospecting techniques commonly are used to aid in the search for and evaluation of subterranean hydrocarbon deposits. A seismic prospecting operation consists of three separate stages: data acquisition, data processing, and data interpretation, and success of the operation depends on satisfactory completion of all three stages.
In the data acquisition stage, a seismic source is used to generate an acoustic signal that propagates into the earth and is at least partially reflected by subsurface seismic reflectors. The reflected signals are detected and recorded by an array of seismic receivers located at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes.
During the data processing stage, the recorded seismic signals are refined and enhanced using a variety of procedures that depend on the nature of the geologic structure being investigated and on the characteristics of the raw data. In general, the purpose of the data processing stage is to produce an image of the subsurface from the recorded seismic data for use during the data interpretation stage.
The purpose of the data interpretation stage is to determine information about the subsurface geology of the earth from the processed seismic data. The results of the data interpretation stage may be used to determine the general geologic structure of a subsurface region, or to locate potential hydrocarbon reservoirs, or to guide the development of an already discovered reservoir.
At present, the conclusions which can be made after the data interpretation stage are generally limited to broad descriptions of the size and general nature of subsurface structures. The descriptions may for example provide an indication of the total volume of hydrocarbons which might be retained in such structures. However, present technology does not allow the analyst to determine preferred locations within a structure for drilling wells, except in a very broad sense. In addition, when an exploration well has been drilled, present technology does not allow an analyst to be able to accurately characterize the nature of the subsurface structure in locations other than in the most immediate region of any such well.
As will be understood from this summary of the data interpretation stage of the seismic prospecting operation, it is desirable to be able to predict the broad nature of a subsurface structure of interest using only seismic data and without having to drill exploration wells. Such a capability would facilitate estimation of hydrocarbon volume in place and production rates early in the hydrocarbon exploration and development process. Hydrocarbon volume and rate of production depend on a variety of factors, including the grain size distribution and thickness of the sand deposit that makes up the reservoir in which the hydrocarbons are found. Accordingly, there is a need for a method for predicting that grain size distribution at every location within a sand deposit and for predicting the three-dimensional size and shape of the deposit. More specifically, there is a need for a method of using a contour of constant deposit thickness to determine the size, shape, and distribution of internal properties, including grain size distribution, in the body. The present invention satisfies that need.